Field of the Invention
Method and device for determining the output power of a semiconductor laser diode that is operated with a diode current.
It is generally known that the output power and the wavelength of the light of a semiconductor laser diode (HLD) are a function of temperature.
In particular, however, it is of decisive importance in optical telecommunications engineering that the output power and wavelength of the light pulses of the used semiconductor laser diode be kept as constant as possible in a very narrow tolerance range. In systems that are operated, for example, using the so-called dense wavelength-division multiplexing (DWDM) method, the spacing between the individual signal channels is only 0.8 nm.
Two preconditions must be fulfilled if both the wavelength and the output power of the light from the semiconductor laser diode are to be stabilized. The first requisite precondition consists in operating the semiconductor laser diode with a constant diode current. Detuning the diode current would lead not only to a change in the output power, but also to a temperature change of the laser-active pn-junction, and would thus entail detuning the wavelength of the light from the semiconductor laser diode.
Following directly from this is the second requisite precondition, namely the stabilization of the temperature of the laser-active region of the semiconductor laser diode.
Various control methods are known for this purpose from the prior art.
One possibility consists in determining the temperature of the semiconductor laser diode with a PTC thermistor or temperature sensor that is arranged in the vicinity of the laser chip, for example, at the edge of the housing of the semiconductor laser diode. The signal generated by the sensor can then, for example, serve for controlling a Peltier element with which the semiconductor laser diode is in thermal contact.
The decisive parameter is, however, the temperature of the laser-active region of the semiconductor laser diode, which can deviate significantly (for example up to 40° C.) from the measured temperature at the edge of the semiconductor laser diode, depending on the geometry of the semiconductor laser diode and the thermal ambient conditions. The essential disadvantage of this indirect measuring method consists in that the semiconductor laser diode can certainly have the same edge temperature for different ambient conditions and diode currents. However, in this case different temperatures can certainly prevail in the laser-active region of the semiconductor laser diode such that the semiconductor laser diode has a different output power and wavelength in conjunction with identical control signals.
A further disadvantage of this method is based on the fact that after a change in the temperature of the Peltier element, the new thermal equilibrium of the semiconductor laser diode is set up only by a certain time constant. For this reason, the control is subject to this time constant, and this can lead to problems with regard to the stability of the temperature control, particularly in the case of semiconductor laser diodes modulated at a high frequency.
A further possibility for temperature control consists in using a monitor diode to monitor the output power of a portion of the emitted light of the semiconductor laser diode. If the output power changes, the measured change signal is output to a control loop. This method certainly has the advantage that changes in the temperature of the laser-active region can be detected without a thermal time constant at the monitor diode, but it is technically complicated and expensive as a function of the operating state of the laser and of applied modulation methods, since it is necessary to integrate an additional measuring diode optically and electronically in the system.